Reflection of a signal 100 (FIG. 1A) in a transmission line, such as a control signal line of a SCSI bus, results in distortions in the signal waveform because the reflected signal is added to or subtracted from the incident signal. FIG. 1A shows an example of a waveform having irregularities caused by reflections during deassertion of an active low signal. Notice that instead of the desired single rising edge on deassertion, signal 100 has multiple rising edges 100A and 100B. Multiple rising edges, which result from reflections of the incident signal, can indicate one or more false deassertions to a circuit receiving signal 100. (See FIG. 1B).
Typically, reflected signals are caused by a mismatch of the impedance between the signal line and either or both of the sending and receiving circuits. To eliminate impedance mismatches, terminators are used on the signal lines. If the termination of a signal line is poor or the terminator is missing, a severe distortion of the incident signal may occur such as that illustrated in FIG. 1B.
One application where a reflected signal may cause spurious events is on a Small Computer System Interface (SCSI) bus. A target device 110, (FIG. 1C) such as, a disk drive, a CD-ROM drive, a printer or a processor, is connected by a SCSI bus 120 to a host computer 130 that includes a host adapter circuit 131 which implements the SCSI protocol. The SCSI protocol is an industry standard interface specification defined in X3.131-1986 available from American National Standards Institute, 1430 Broadway, New York, N.Y. 10018. Document X3.131-1986 is incorporated herein in its entirety.
A SCSI bus has two sets of signal lines. A first set of signal lines is used to transfer control signals between target device 110 and host adapter circuit 131. A second set of signal lines is used to transfer data between target device 110 and host adapter circuit 131. If the termination on any one of the control signal lines is defective or even missing, signal waveforms such as those shown in FIGS. 1A and 1B may occur. The false edges created by the reflected signals may appear as true edges to host adapter circuit 131. In this case, the SCSI bus protocol is corrupted by the false edges. Typically, various methods have been used to provide an effective termination on the control signal lines. Nevertheless, reflected signals on the SCSI bus remain a problem.